compared, by ascertaining the greatest length of each of the substances to be examined, through which a spark or a shock will take its course, in preference to a given length of air, or of any other standard of comparison. The substances which conduct electricity the most readily, are metals, well burnt charcoal, animal bodies, acids, saline liquors, water, and very rare air. The principal nonconductors are glass, ice, gems, dry salts, sulphur, amber, resins, silk, dry wood, oils, dry air of the usual density, and the barometrical vacuum. Heat commonly increases the conducting powers of bodies; a jar of glass may be discharged by a moderate heat, and liquid resins are capable of transmitting shocks, although they are by no means good conductors: it is remarkable also that a jar may be discharged by minute agitation, when it is caused to ring by the friction of the finger. It has been observed that, in a great variety of cases, those substances which are the best conductors of heat, afford also the readiest passage to electricity; thus, copper conducts heat more rapidly, and electricity more readily, than iron; and platina less than almost any other metal glass also presents a considerable resistance to the transmission of both these influences. The analogy is, however, in many respects imperfect, and it affords us but little light, with regard either to the nature of heat, or to that of the electric fluid. [Young's Nat. Phil.

SECTION II1.

Electricity in Motion.

THE manner in which the electric fluid is transferred from one body to another, the immediate effects of such a transfer, the causes which originally disturb the equilibrium of electricity, and the practical methods by which all these circumstances are regulated and measured, require to be considered as belonging to the subject of electricity in motion. Among the modes of excitation by which the equilibrium is originally disturbed, one of the most interesting is the galvanic apparatus, which has been of late years a very favourite subject of popular curiosity, and of which the theory and operation will be briefly examined, although the subject appears rather to belong to the chemical than to the mechanical doctrine of electricity.

The progressive motion of the electric fluid through conducting

substances is so rapid, as to be performed in all cases without a sensible interval of time. It has indeed been said, that when very weakly excited, and obliged to pass to a very great distance, a perceptible portion of time is actually occupied in its passage; but this fact is somewhat doubtful, and attempts have been made, in vain, to estimate the interval employed in the transmission of a shock through several miles of wire. We are not to imagine that the same particles of the fluid which enter at one part, pass through the whole conducting substance, any more than that the same por. tion of blood which is thrown out of the heart, in each pulsation, arrives at the wrist at the instant that the pulse is felt there. The velocity of the transmission of a spark, or shock, far exceeds the actual velocity of each particle, in the same manner as the velocity of a wave exceeds that of the particles of water concerned in its propagation; and this velocity must depend both on the elasticity of the fluid, and on the force with which it is confined to the con ducting substance. If this force were merely derived from the pressure of the atmosphere, we might infer the density of the fluid from the velocity of a spark or shock, compared with that of sound; or we might deduce its velocity from a determination of its density. It has been supposed, although perhaps somewhat hastily, that the actual velocity is nearly equal to that of light.

When a conducting substance approaches another, which is electrified, the distribution of the electric fluid within it is necessarily altered by induction, before it receives a spark, so that its remoter extremity is brought into a state similar to that of the first body hence it happens that when the spark passes, it produces less effect at the remoter end of the substance, while the part presented to the electrified body is most affected, on account of its sudden change to an opposite state. But if both ends approach bodies in opposite states of electricity, they will both be strongly affected when the shock takes place, while the middle of the cir. cuit undergoes but little change.

The manner in which the electric fluid makes its way through a more or less perfect conductor, is not completely understood: it is doubtful whether the substance is forced away on each side, so as to leave a vacuum for the passage of the fluid, or whether the newly formed surface helps to guide it in its way; and in some cases it has been supposed, that the gradual communication of elec. tricity has rendered the substance more capable of conducting it,

either immediately, or, in the case of the air, by first rarefying it. However this may be, the perforation of a jar of glass by an overcharge, and that of a plate of air by a spark, appear to be effects of the same kind, although the charge of the jar is princi. pally contained in the glass, while the plate of air is perhaps little concerned in the distribution of the electricity.

The actual direction of the electric current has not in any instance been fully ascertained, although there are some appearances which seem to justify the common denominations of positive and negative. Thus, the fracture of a charged jar of glass, by spontaneous explosion, is well defined on the positive, and splintered on the negative side, as might be expected from the passage of a foreign substance from the former side to the latter; and a candle, held between a positive and a negative hall, although it apparently vibrates between them, is found to heat the negative ball much more than the positive. We cannot, however, place much depend. ance on any circumstance of this kind, for it is doubtful whether any current of the fluid, which we can produce, possesses suffi. cient momentum to carry with it a body of sensible magnitude. It is in fact of little consequence to the theory, whether the terms positive and negative be correctly applied, provided that their sense remain determined; and that, like positive and negative quantities in mathematics, they be always understood of states which neutralise each other. The original opinion of Dufay, of the existence of two distinct fluids, a vitreous and a resinous electricity, has at present few advocates, although some have thought such a supposition favoured by the phænomena of the galvanic decomposition of water.

When electricity is simply accumulated without motion, it does not appear to have any effect, either mechanical, chemical, or physiological, by which its presence can be discovered; the acce. leration of the pulse, and the advancement of the growth of plants, which have been sometimes attributed to it, have not been confirmed by the most accurate experiments. An uninterrupted cur. rent of electricity, through a perfect conductor, would perhaps be also in every respect imperceptible, since the best conductors appear to be the least affected by it. Thus, if we place our hand on the conductor of an electrical machine, the electricity will pass off continually through the body, without exciting any sensation. A constant stream of galvanic electricity, passing through an iron

wire, is, however, capable of exciting a considerable degree of heat; and if it be transmitted through the hands of the operator, it will produce a slight numbness, although in general some interruption of the current is necessary, in order to furnish an accumulation sufficient to produce sensible effects; and such an interruption may even increase the effect of a single spark or shock; thus, gunpowder is more readily fired by the discharge of a battery passing through an interrupted circuit, than through a series of perfect conductors.

The most common effect of the motion of the electric fluid is the production of light. Light is probably never occasioned by the passage of the fluid through a perfect conductor; for when the discharge of a large battery renders a small wire luminous, the fluid is not wholly confined to the wire, but overflows a little into the neighbouring space. There is always an appearance of light whenever the path of the fluid is interrupted by an imperfect conductor; nor is the apparent contact of conducting substances suffi cient to prevent it, unless they are held together by a considerable force; thus, a chain, conveying a spark or shock, appears lumi. nous at each link, and the rapidity of the motion is so great, that we can never observe any difference in the times of the appearance of the light in its different parts; so that a series of luminous points, formed by the passage of the electric fluid, between a string of conducting bodies, represents at once a brilliant delinea. tion of the whole figure in which they are arranged. A lump of sugar, a piece of wood, or an egg, may easily be made luminous in this manner; and many substances, by means of their properties as solar phosphori, retain for some seconds the luminous appearance thus acquired. Even water is so imperfect a conductor, that a strong shock may be seen in its passage through it; and when the air is sufficiently moistened or rarefied to become a conductor, the track of the fluid through it is indicated by streams of light, which are perhaps derived from a series of minute sparks passing between the particles of water, or of rarefied air. When the air is extremely rare, the light is greenish; as it becomes more dense, the light becomes blue, and then violet, until it no longer con. ducts. The appearance of the electrical light of a point enables us to distinguish the nature of the electricity with which it is charged; a pencil of light, streaming from the point, indicating that its electricity is positive; while a luminous star, with few

diverging rays, shows that it is negative. The sparks, exhibited by small balls, differently electrified, have also similar varieties in their forms, according to the nature of their charges.

The production of heat by electricity frequently accompanies that of light, and appears to depend in some measure on the same circumstances. A fine wire may be fused and dissipated by the discharge of a battery; and without being perfectly melted, it may sometimes be shortened or lengthened, accordingly as it is loose or stretched during the experiment. The more readily a metal conducts, the shorter is the portion of it which the same shock can destroy; and it has sometimes been found, that a double charge of a battery has been capable of melting a quadruple length of wire of the same kind.

The mechanical effects of electricity are probably in many cases the consequences of the rarefaction produced by the heat which is excited; thus, the explosion attending the transmission of a shock or spark through the air, may easily be supposed to be derived from the expansion caused by heat; and the destruction of a glass tube, which contains a fluid in a capillary bore, when a spark is caused to pass through it, is the natural consequence of the conversion of some particles of the fluid into vapour. But when a glass jar is perforated, this rarefaction cannot be supposed to be adequate to the effect. It is remarkable that such a perforation may be made by a very moderate discharge, when the glass is in contact with oil or with sealing wax; and no sufficient explanation of this circumstance has yet been given.

A strong current of electricity, or a succession of shocks or sparks, transmitted through a substance, by means of fine wires, is capable of producing many chemical combinations and decom. positions, some of which may be attributed merely to the heat which it occasions, but others are wholly different. Of these the most remarkable is the production of oxygen and hydrogen gas from common water, which are usually extricated at once, in such quantities, as, when again combined, will reproduce the water which has disappeared; but in some cases the oxygen appears to be disengaged most copiously at the positive wire, and the hydrogen at the negative.

When the spark is received by the tongue, it has generally a subacid taste; and an explosion of any kind is usually accompanied by a smell somewhat like that of sulphur, or rather of fired